This chapter is an update to the earlier Science of Synthesis contribution (Section 15.9.3), covering selected methods for the preparation and the diverse fields of application of acridinium salts. The most important classical and recently published routes toward acridinium core construction are described and categorized according to key retrosynthetic disconnections. The utility of acridinium moieties in supramolecular chemistry is showcased by examples for various supramolecular switches containing this heterocyclic system. The application of acridinium salt derivatives as chemosensors for the detection of anionic species is also shown. Furthermore, the chapter features recent representative methods within the field of photoredox catalysis using acridinium salts as photocatalysts.